The present disclosure discloses a horizontal-axis ocean current power generation device for an underwater vehicle. The power generation device is disposed in a groove of a rotary body of the underwater vehicle, and includes an undercarriage unit, a yawing unit, and a power generation unit. The undercarriage unit can realize elevation and descent of the entire power generation device, and the power generation unit is capable of realizing arbitrary rotation within 360° in a horizontal plane through the yawing unit. The power generation device can actively yaw based on change of an ocean current direction to perform an incident flowing function. The power generation unit respectively drives an outer shaft and an inner shaft to rotate through a front blade and a rear blade that rotate in opposite directions, so as to drive inner and outer rotors of a motor, thereby cutting magnetic induction to generate electric power.